MySQL row locks and gap locks

Whether it is update or select for update, as long as the field in the where condition does not have an index, the entire table will be locked.If an index exists on the updated row, the updated record is locked when updating.

1, row lock

Test:
Item table, lock on the id and price fields.

Opens two windows.Turn off autocommit separately: set autocommit=0;
 

1.1, the field has an index, row lock

a window execution:

update item2 set stat = 1 where price = 1500;

bWindow execution:

update item2 set stat = 1 where price = 30;

Since there is an index in the price field, the update statement of the a windowOnly one record with price=1500 is locked.The b window normally updates the record with price=30.
 

1.2, there is no index in the field, table lock

a window execution:

update item2 set stat = 1 where name= 'dd';

bWindow execution:

update item2 set stat = 1 where price = 'ee';

Since the name field has no index, the a window is limited wherename='dd', before the commit, the query of the record with name='ee' in the b window is still blocked.So if the field does not have an index, a table lock will be performed on update.

 

1.3, the case of joint index

Create a new joint index on the three fields of name, address, and port

create index index_name_address_port on item2(name,address,port);

1.3.1, when the joint index is valid

a window execution:

update item2 set stat=1 where name = 'cc4' and address='Zhangjia Village';

bWindow execution:

 update item2 set stat=1 where name = 'ff' and address='bcbc';

As long as the where query index is valid, the joint index will still onlyLocking a row does not affect updates of other rows.
 

1.3.2, the joint index does not take effect

a window execution:

update item2 set stat=1 where address='Zhangjia Village' and port='67';

bWindow execution:

update item2 set stat=1 where address='bcbc' and port='24';

If the joint index in the where condition is invalid, it will still beLock the entire table.

Joint indexing conditions

Mysql uses the fields in the index from left to right. A query can only use a part of the index, but only the leftmost part.For example the index is key index (a,b,c).Can support a | a, b | a, b, c 3 combinations for search, but do not support b, c for search. When the leftmost field is a constant reference, the index is very effective.

 
 

2, gap lock

A special case of row locks: gap locks: value in range, but not present
a window execution:

 update item2 set stat = 1 where price>100 and price<400;

bWindow execution

 INSERT INTO `item2` VALUES (12, 'gt', 'vxc', 234, 288, NULL, NULL);

c window execution:

update item2 set stat = 1 where price = 300;

After a window is executed, b and c windows are blocked.The records indicating (100, 400) are locked.Unable to insert records with price before 100-400, even if the record with price=288 does not exist yet, magic degree can be avoided.There is also no way to update records that already existed before 100-400.